Many players hurt themselves on the greens, wasting strokes by three-putting. The root of their problems may be an inability to judge how hard to hit long putts.
Unfortunately, there are no distance markers on the green; that doesn't mean knowing the distance of long putts is unimportant. You should pace off long putts and have practiced enough of them to have developed a feel for the correct stroke.

To pace off a putt, multiply the number of
your paces by three (assuming your stride is about
standard -- three-feet long) to come up with the
approximate length in feet. Remember, you're looking for
a ball-park figure, not an exact number, work quickly so
you don't slow down play. (In fact, do your measuring
when you first arrive at the green, not when the rest of
your group is waiting for you to line up and stroke.)

Learn the proper stroke on the practice
green: Pace off a 25-foot putt and putt a few balls,
making a mental note of the length and force of the
stroke. Move back and try some from 30, then 35, 40, 45
and 50 feet. During play, pace off any putt that's too
long for your "comfort zone." You'll draw a lot of
confidence on the greens by calculating the distance,
especially if you've spent the time practicing from
different lengths. Stick to this plan and three-putting
the long ones will become a thing of the past. Besides
that, there's the added bonus of sometimes sinking a
monster.
Private Lessons 1999 Time4 Media, Inc., used under license by GolfServ Online, Inc. Instructional information provided by Golf Magazine.